I have studied SiN nucleation on Si (111) 7x7 surface by thermal nitridation in vacuum and its masking effects against thermal oxidation for Si quantum dot formation. As oxidation conditions, etching mode oxidation in vacuum and film growth mode oxidation in a furnace were employed. As a result, following facts were found.1. The nitrides, of which the thickness is about 0.5nm, have strong immunity against etching mode oxidation, resulting Si pillar formation. The height linearly increases with oxidation time.2. The nitrides also work as oxidation masks for furnace-oxidation by taking the sample out of vacuum to the air. This result indicates that the proposed process is useful and effective in pillar and dot formation.3. The size and density of pillars are corresponding to those of SiN nuclei and nm-scale structure is obtained by nitridation in a relatively low temperature range of 600-650C.The next step is to apply this process to SOI substrates for fabricating Si dot arrays, which is being studied.